| The widely exposed diabase veins,lamprophyre veins and granodiorite porphyry veins in Beishan area Gansu,provide important information for the tectonic evolution of Beishan area and its adjacent areas.On the basis of detailed field geological survey,this paper selects the diabase vein,lamprophyre vein and granodiorite porphyry vein exposed in Yinwaxia and Xinchang areas in the south of Beishan and intruded into the late Paleozoic granites as the research object.Through the main and trace element and Sr Nd isotope geochemical characteristics of the three types of vein rocks and LA-ICP-MS zircon U-Pb dating technology,it reveals the petrogenesis,tectonic background and geological significance of the formation of the three types of dike rocks.The main achievements are as follows:1.The LA-ICP-MS zircon U-Pb isotope age of the Yinwaxia diabase vein is235.9±1.9 Ma,which was formed in the Late Triassic.The rock has low Si O2（49.10-53.47 wt%）,Al2O3（13.12-14.94 wt%）,high TFe2O3（7.25-10.82 wt%）,Mg O（4.13-5.05 wt%,Mg#40.50-55.42）;Na2O（2.71-3.53 wt%）,K2O（1.43-1.93 wt%）,Na2O>K2O,total alkali（Na2O+K2O=4.49-5.18 wt%）,Rittmann indexσ2.10-3.93.Light rare earth elements are relatively enriched,while heavy rare earth elements are relatively depleted;Weak negative Eu anomaly,δEu values range from0.80 to 0.90;The sample is deficient in Ba,Nb,Ta,Ce,and Sr,enriched in U,K,and Pb,with La/Ta values ranging from 29.92 to 30.71 and Nb/U values ranging from3.19 to 5.45.The research shows that diabase is derived from the partial melting of lithospheric enriched mantle previously metasomatized by subduction fluid,and is the product of rapid rise emplacement of magma,formed in the tectonic environment of intraplate extension.2.The LA-ICP-MS zircon U-Pb isotope age of the Yinwaxia lamprophyre vein is 237.1±1.9 Ma,which was formed in the Middle Triassic.The lamprophyre has high Si O2（45.66-64.88 wt%）,Al2O3（13.36-15.40 wt%）,K2O（2.12-3.15 wt%）,Na2O（2.03-4.50 wt%）,The ratios of K2O/Na2O（0.63-1.26）,relatively rich in sodium and poor in potassium;Total alkali（Na2O+K2O）ranges from 4.60 to 7.85 wt%,Mg O（0.92 to 9.51 wt%,Mg#=23.42 to 64.61）,Ca O（2.99-9.18 wt%）,P2O5（0.25-0.33wt%）,Ti O2（1.22-1.66 wt%）,and Rittmann indexσranges from 1.64 to 7.95,with an average of 3.99.The above characteristics indicate that the Yinwaxia lamprophyre vein is weakly potassic calc alkaline lamprophyre.The total amount of rare earth elements（∑REE）in rocks ranges from 207.03-264.95 ppm,the total amount of light rare earth elements（∑LREE）ranges from 119.01-163.42 ppm,the total amount of heavy rare earth elements（∑HREE）ranges from 88.02 to 98.98 ppm,and the∑LREE/∑HREE ranges from 1.32 to 1.65.Light rare earth elements are relatively abundant,while heavy rare earth elements are relatively depleted.（La/Yb）N=3.99-4.43,indicating varying degrees of differentiation between light rare earth elements and heavy rare earth elements.（La/Sm）N=2.32-2.64,（Gd/Yb）N=1.38-1.42,weak negative Eu anomaly,δEu ranges from 0.71 to 0.78.The rocks have varying degrees of depletion of high field strength elements（Nb,Ta,Ce,Sr,P,Sm,and Ti）and Ba,while enrichment of large ion lithophile elements（Rb and K）as well as Th,U,and Pb;(87Sr/86Sr)iis between 0.708544 and 0.709328,εNd（t）value is（-2.59～-4.04）,and the geochemical characteristics of the Nb/U value（2.82-3.26）close to the subduction fluid.The results show that the lamprophyre vein was produced by the low degree partial melting（4%-8%）of the lithospheric enriched mantle previously metasomatized by subduction fluid,and was formed in a post collision extensional tectonic environment.3.The LA-ICP-MS zircon U-Pb isotope age of the Xinchang granodiorite porphyry vein is 241.2±1.2 Ma,formed during the Middle Triassic.The rock has high Si O2（71.43-73.99 wt%）,K2O（1.24-5.13 wt%）,Na2O（4.40-5.52 wt%）,K2O/Na2O value is 1.31-1.56,（K2O+Na2O=6.52-9.78 wt%）,low Ca O（1.47-1.70 wt%）,low Mg O（0.36-0.38 wt%,Mg#27.85-28.92）,low P2O5（0.07-0.08wt%）,and low Ti O2（0.35-0.39 wt%）,The A/CNK value is 1.17-1.71.10000Ga/Al value is（2.85-3.47）>2.6,and the magma temperature during rock formation is897-926℃,with geochemical characteristics of A-type granite.The rocks are enriched in Rb,Th,U,Pb,and K,and depleted in Ba,Nb,Ta,P,Ti;The Th/U value is2.89-8.49,the Nd/Th value is 2.29-2.38,and the(87Sr/86Sr)ivalue is 0.707036-0.707311,εNd（t）ranges from-1.22 to-0.48,and the TDM2of Nd is 1062-1115 Ma.The comprehensive research results show that the granodiorite porphyry should be the product of partial melting of the late Mesoproterozoic new lower crust under the heating of mantle basaltic magma underplating,and formed in a post collision extensional tectonic environment.4.Combined with the regional geological characteristics,the diabase veins,lamprophyre veins and granodiorite porphyry veins in the study area were all formed in the intraplate extensional environment of the Middle Triassic,indicating that at least before the Middle Triassic,the Beishan area had completed the transformation of the oceanic continental tectonic system and entered the intraplate extensional tectonic evolution stage. |
